RAFAH, Gaza (CNN) --An Israeli bulldozer killed an American woman Sunday who had been protesting its use to destroy Palestinian houses in Rafah.

The woman, Rachel Corrie, 23, of Olympia, Washington, was taken to a hospital, where she died of her injuries. She was a senior at Evergreen State College in Olympia but was not enrolled this quarter, the school said.

Since January, she had been working with the Palestinian-led International Solidarity Movement to protest Israeli actions in the occupied territories, said Huwaida Arraf, co-founder of the group.

Corrie had recently appeared in a televised mock trial in Gaza in which President Bush was accused of war crimes for his alleged support of Israel's actions in Gaza.

"This morning, when she was killed, she was attempting to prevent the Israeli military from destroying Palestinian civilian homes," Arraf said.

"She was raising her hands and yelling at the bulldozer driver to stop," Arraf said. "The bulldozer driver paid no attention. ... He buried Rachel with dirt, which ended up, obviously, knocking her down. Then he ran over her, and then reversed and ran over her again."

Other witnesses, however, reported that Corrie had scaled a pile of dirt but then lost her footing and fell backward behind it, out of sight of the bulldozer operator. The bulldozer continued moving forward, covering Corrie with dirt and then crushing her.

It was not clear whether the bulldozer operator could hear protesters' yells over the sound of the machine.

A member of the solidarity group, who identified herself as Alice from London, said she and Corrie had sat for about three hours in front of houses belonging to their friends. The driver of the bulldozer must have seen them, she said, but drove over Corrie anyway.

She emerged from under the bulldozer saying, "My back is broken, my back is broken," Alice told CNN.

Tom Dale, who said he was about 10 yards from Corrie, said she was in plain view and was wearing an orange jacket. As the bulldozer lifted a pile of earth, it moved forward and caught Corrie under its blade, he said.

Israel: 'Very regrettable incident'

"This is a very regrettable incident," an Israeli military source said. "This is a group of protesters who are acting very irresponsibly. They are putting everyone in danger, the Palestinians, themselves, our forces, by intentionally placing themselves in a combat zone. We are checking the details of the incident and believe it to be a very regrettable incident."

Arraf said the activists use only nonviolent tactics. "We definitely don't believe that this was an accident," she said.

Corrie's parents, who live in Charlotte, North Carolina, said their daughter felt an obligation to help others.

"I've raised my children to be independent and to make their own choices, and I knew that I couldn't tell her not to go," said her mother, Cindy Corrie.

"We were very proud of her," said Craig Corrie, her father. "We're very proud of her courage and what she stood for, and we're very proud of Rachel. She's 23 years old, and while that seems young to me, it's old enough for her to make up her own mind about what she wants to do. There's no holding her back."

The U.S. State Department said it was in contact with Corrie's family.

"The United States deeply regrets this tragic death of an American citizen," spokesman Lou Fintor said. "We offer our sincere condolences to Ms. Corrie's family."

Fintor said the United States urged Israel and the Israel Defense Forces to conduct "an immediate and full investigation into the circumstances of this death."

The United States also repeated its call for the IDF to take all possible measures to avoid harming civilians, Fintor said.